This invention relates to ring laser gyroscopes and more particularly, the invention relates to the absorption of reflected light energy or electromagnetic waves from intracavity elements such as a Faraday rotator.
One of the most significant ring laser gyroscopes yet proposed and constructed employs four waves of two pairs each propagating in opposite directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657, 3,854,819 and 4,006,989 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such laser systems, circular polarization for each of the four waves is used. The pair of waves, or beams, propagating in the clockwise direction includes both left-hand circularly polarized (LCP) waves and right-hand circularly polarized (RCP) waves as do those waves propagating in the counter-clockwise direction. This four-frequency or multi-oscillator ring laser gyro provides a means of circumventing the frequency locking or lock-in problem present in all conventional or two-frequency laser gyroscopes. This lock-in phenomenon occurs when two traveling waves propagating in opposite directions in a resonant cavity at slightly different frequencies are pulled toward each other to combine in a single frequency standing wave. However, when the frequencies of the counter-rotating waves are sufficiently separated in frequency, the pulling together does not occur. The four-frequency approach may be described as two independent laser gyros operating in a single stable resonator cavity, sharing a common optical path, but statically biased in opposite senses by the same passive bias element. In the differential output of these two gyros, the bias then cancels, while any rotation-generated signals add, thereby avoiding the usual problems due to drifts in the bias and giving a sensitivity twice that of a single two-frequency gyro. Because the bias need not be dithered, the gyro never passes through lock-in. Hence, there are no dither-induced errors to limit instrument performance. For this reason, the four frequency gyro is intrinsically a low noise instrument, and it is well suited for applications requiring rapid position update or high resolution.
The four different frequencies are normally generated by using two different optical effects. First, a crystal polarization rotator may be used to provide a direction-independent polarization causing the resonant waves to be circularly polarized in two directions. The polarization rotation results from the refractive index of the rotation medium being slightly different for RCP and LCP waves. Alternatively, a non-planar ring path may be used which inherently supports only circularly polarized waves without the use of a crystal rotator. A non-planar electromagnetic wave ring resonator is shown and described in U.S. Pat. No. 4,110,045 to Irl W. Smith, Jr. and Terry A. Dorschner and assigned to the present assignee. Second, a Faraday rotator is used to provide nonreciprocal polarization rotation, by having a slightly different refractive index for clockwise (cw) traveling waves than for counter-clockwise (ccw) traveling waves. This causes the cw and ccw RCP waves to oscillate at slightly different frequencies while the cw and ccw LCP waves are similarly but oppositely split. Thus, a laser gyro operates with right circular polarized waves biased in one direction of rotation and with left circular polarized waves biased in the opposite direction, the bias being cancelled by subtracting the two outputs.
Although a Faraday rotator provides non-reciprocal polarization rotation and has anti-reflection coating on both sides of its glass material, its insertion into the optical path results in some light energy being back-reflected by the rotator. In order to prevent these reflections from mixing with the main cw and ccw propagating waves, they must be absorbed or reflected away from the main propagating waves.